<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<div id="demo"></div>
<script>
    document.getElementById("demo").style.width ="200px";
    document.getElementById("demo").style.height ="200px";
    document.getElementById("demo").style.backgroundColor ="red";
</script>
</body>
</html>